1. Field of the Invention
The present invention relates to an endoscope system that illuminates the inside of a body cavity using a plurality of semiconductor light sources such as LEDs, and an operating method thereof.
2. Description Related to the Prior Art
In a medical field, diagnosis using an endoscope system having a light source unit, an endoscope, and a processor device is widely carried out. The endoscope is generally provided with a color image sensor, which images an observation object and outputs an image signal of a plurality of colors (a multi-colored image signal). Since the difference among the image sensors varies the spectral sensitivity of the endoscopes, it is known that the variations in the spectral sensitivity cause color variations among endoscopic images.
As a method for eliminating such color variations among the endoscopic images, there is a method by which whenever exchanging the endoscope, a white reference object is imaged with the color image sensor and the multi-colored image signal is obtained. Then, a gain process is applied thereto by multiplying an image signal of each color by a gain coefficient such that the multi-colored image signal attains a predetermined balance, in order to adjust white balance of the endoscopic image. Also, provided that the light source unit includes a plurality of semiconductor light sources that can emit light of three colors i.e. red light, green light, and blue light in such a manner as to independently control individual light amounts, as described in Japanese Patent Laid-Open Publication No. 2002-122794, there is another method for adjusting the white balance of the endoscopic image by which the amount of light of each color is controlled such that the multi-colored image signal, which is obtained by imaging the white reference object with the color image sensor, attains the predetermined balance.
Here, if the image sensor is an RGB color sensor having favorable color separability in which light of different colors is hard to mix in pixels of a certain color (for example, a sensor in which blue light and red light are hardly mixed in G pixels), the white balance can be adjusted just by independently controlling the amount of light of each color, without increasing the gain coefficients of the gain process. Not increasing the gain coefficients minimizes noise.
On the other hand, in a case where the image sensor has poor color separability in which light of different colors is easy to mix in pixels of a certain color (for example, a sensor in which blue light and red light are easily mixed in G pixels), the adjustment of the white balance just by controlling the amount of light of each color disturbs the balance among the amounts of light of individual colors. Taking a case where G pixels are sufficiently sensitive to blue light and red light (see FIG. 5) as an example, adjusting the white balance just by controlling the amount of light of each color makes the amount of green light smaller than the amounts of the blue light and the red light, as shown by a solid line in FIG. 10. Note that, in FIG. 10, the solid line represents relative radiant intensity of the blue light, the green light, and the red light after the adjustment of the white balance by the light amount control. A dashed line represents default relative radiant intensity of the blue light, the green light, and the red light.
As described above, in a case where the amount of the green light becomes smaller than the amounts of the blue light and the red light, color may not be correctly displayed in the image. Accordingly, a color correction matrix process is applied with the aim of correctly displaying the color of the image. In this case, however, color correction coefficients (especially, a color correction coefficient for correcting a green component) used in the color correction matrix process become high. The high color correction coefficients increase noise.